At present, several different, and incompatible, mobile communications specifications exist for different territories; for example, the GSM specification in Europe and elsewhere, and the D-AMPS specification in the US. Although a mobile terminal which conforms to one specification may be able to operate with different networks provided they all conform to that specification and they have so-called "roaming" agreements, the mobile terminal is not able to operate with networks which conform to a different specification. Thus, a GSM phone will not be useable in many parts of the US.
Mobile telephony networks such as GSM consist of base transceiver stations (BTSs) which are in radio communication with the mobiles; base control stations (BSCs) connected to the BTSs, and which manage low level signalling control functions such as local handover; and mobile switching centres (MSCs) which act as switching exchanges, and are usually co-located with mobility management databases.
These MSC's, together with the physical links (e.g. fibre optic cables) which interconnect them, make up a backbone network, through which voice calls, fax calls or data exchanges (hereafter collectively termed "sessions") are routed after leaving the air interface at the BTS. The signalling protocols used on the backbone network are specified within the GSM specification, and bear some similarity to those of the integrated services digital network (ISDN).
One prior proposal is disclosed in our earlier application WO 96/28947, which describes a multi-mode universal mobile telecommunications system, in which a multiple access satellite communication system is provided to co-operate with multimode terminals which can operate according to several different communications standards. In this case, both high level and low level protocols employed, and even the radio frequencies employed, by the various different terrestrial and satellite communications systems envisaged, may differ between the different systems. Other aspects of our earlier proposal may be employed together with the present invention as described hereafter.
At present, discussions are taking place on the future generation of mobile communications systems, termed the universal mobile telecommunications system (UMTS), but no standards or specifications have been settled. It appears likely that the UMTS will specify a radio interface and a radio access network, but will not specify a unique backbone network as GSM does.